Climatic and tectonic controls on Triassic dryland terminal fluvial system architecture, central North Sea - From Depositional Systems to Sedimentary Successions on the Norwegian Continental Margin - page 20

INTRODUCTION

Central North Sea stratigraphic framework

The architecture and preservation of terminal,
dryland fluvial systems such as the Triassic in
the central North Sea are relatively under-
researched compared to that of paralic fluvial
systems where relative sea-level fluctuations
exert an over-arching control (Shanley &
McCabe, 1994; Blum & Törnqvist, 2000). Early
models of the sequence stratigraphy of such
hinterland successions (e.g. Steel & Ryseth,
1990; McKie & Garden, 1996; Legarreta & Uliana,
1998) emphasised the predictable expansion
and contraction of generally terminal fluvial
systems in response to changes in the accom-
modation: supply ratio of the depositional sys-
tem and fluctuations in stratigraphic base level
(Martinsen et al ., 1999), but lacked the regional
palaeoclimatic information to disentangle the
complex fluvial response to basinal and catch-
ment controls on base level and sediment yield
(Blum & Törnqvist, 2000). The aim of this paper
is to explore the multivariate controls on the
alluvial architecture of the central North Sea
Triassic by placing the evolution of these termi-
nal fluvial depositional systems within a wider
framework of the tectonics and climate change
which were occurring on the northern Tethyan
margin of Pangaea, and to examine their relative
contribution on the source-to-sink fluvial rout-
ing into the central North Sea region. Although
the Triassic in the central North Sea has limited
well penetrations, a patchy preservation (Fig. 1)
and relatively low-resolution chronostrati-
graphic framework, the Southern Permian Basin
immediately to the south, with its classical
'Germanic' succession of Bunter, Muschelkalk
and Keuper formations, is relatively well
understood (Aigner & Bachmann, 1992; Geluk,
2005; Bachmann et al ., 2010) and provides a
robust, down-palaeoflow framework of tecton-
ics, climate change and marine influence which
can be extended both southward into the marine
Tethyan margin and northward into the fully
terrestrial central North Sea area (Michelsen &
Clausen, 2002). The overall stratigraphic evolu-
tion between these regions can be compared and
similarities and departures from the regional
climate and tectonic setting used to make infer-
ences on the controls on deposition and preser-
vation in the generally endorheic central North
Sea region.

The stratigraphic subdivision of the Triassic
across the North Sea region varies both by basin
and across international boundaries (Lervik,
2006). In the UK central North Sea (Fig. 2) the
Triassic succession is generally divided into the
mud-prone Smith Bank and overlying, sand-
prone, Skagerrak formations (Cameron et al .,
1992). These subdivisions can be correlated
locally into the Norwegian (Lervik, 2006) and
Danish (Michelsen & Clausen, 2002) sectors in the
area to the north of the mid-North Sea and
Ringkøbing-Fyn highs (Fig. 1B). South of these
highs, in the Southern Permian Basin, the more
marine influenced Germanic succession prevails
(Bertelsen, 1980; Michelsen & Clausen, 2002;
Geluk, 2005; Geluk, 2007). In the UK sector of the
central North Sea the Skagerrak Formation has
been further subdivided into biostratigraphically
constrained, sand-prone and mud-prone members
(Fig. 2; Goldsmith et al ., 1995, 2003), but in the
more sparsely penetrated Norwegian sector these
subdivisions are locally more difficult to identify.
Biostratigraphic recovery is generally poor, par-
ticularly in the Early Triassic to Anisian (Smith
Bank and Bunter Sandstone formations and lower
Judy Sandstone Member), but the Late Anisian-
Early Ladinian and Late Ladinian intervals are
locally highly productive (Goldsmith et al ., 1995;
Lindström et al ., 2009), allowing the regional
correlation of the upper Judy Sandstone, Julius
Mudstone and lower Joanne Sandstone members.
The Joanne Sandstone Member forms a wide-
spread alluvial package, but this has been variably
eroded by an intra-Skagerrak Formation uncon-
formity (Archer et al ., 2010). The uppermost parts
of the Triassic succession, corresponding to the
Jonathan Mudstone, Josephine Sandstone and
Joshua Mudstone members (Fig. 2), are also very
poorly preserved due to multiple Jurassic ero-
sional events and are only well developed in UK
blocks 30/7, 30/12 and 30/13 (Goldsmith et al .,
1995). Equivalent successions may be present in
the Norwegian and Danish sectors, but these
can lack the well-defined shale packages which
differentiate individual members.
Whilst a robust stratigraphic succession has
been defined in key areas where palynologically
productive horizons are preserved, correlation of
the Skagerrak Formation members is hampered
by the generally poor biostratigraphic recovery,